Semiconductor device and manufacturing method thereof

ABSTRACT

On the top surface of a thin semiconductor wafer, top surface structures forming a semiconductor chip are formed. The top surface of the wafer is affixed to a supporting substrate with a double-sided adhesive tape. Then, from the bottom surface of the thin semiconductor wafer, a trench, which becomes a scribing line, is formed by wet anisotropic etching so that side walls of the trench are exposed. On the side walls of the trench with the crystal face exposed, an isolation layer with a conductivity type different from that of the semiconductor wafer for holding a reverse breakdown voltage is formed simultaneously with a collector region of the bottom surface diffused layer by ion implantation, followed by annealing with laser irradiation. The side walls form a substantially V-shaped or trapezoidal-shaped cross section, with an angle of the side wall relative to the supporting substrate being 30-70°. The double-sided adhesive tape is then removed from the top surface to produce semiconductor chips. With such a manufacturing method, a reverse-blocking semiconductor device having high reliability can be formed.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of Ser. No. 11/389,495, which is acontinuation-in-part of U.S. patent application Ser. No. 11/208,459filed 19 Aug. 2005, the entire disclosures of which are incorporatedherein by reference.

BACKGROUND

In a semiconductor device of a reverse-blocking type, a reverse-blockingcapability equivalent to a forward-blocking capability is required. Forsecuring the reverse-blocking capability, it is necessary to make a p-njunction holding a reverse breakdown voltage to extend from the bottomsurface of a semiconductor chip to its top surface. A diffused layer forforming the p-n junction extending from the bottom surface to the topsurface is an isolation layer.

FIG. 10 illustrates the manufacturing steps of forming an isolationlayer in a related reverse-blocking IGBT presenting its principal part.The method is that of forming the isolation layer by coating anddiffusion. First, on a semiconductor wafer 1, an oxide film 2 having afilm thickness of the order of 2.5 μm is formed by thermal oxidation asa dopant diffusion mask (step (a)). Next, the oxide film 2 is subjectedto patterning and etching, by which an opening 3 with a diameter of theorder of 100 μm is formed for forming an isolation layer (step (b)).Thereafter, the opening 3 is coated with a boron source 4 and hightemperature and long time heat treatment of the semiconductor wafer 1 iscarried out in a diffusion furnace to form a p-type diffused layer witha thickness of the order of several hundreds micrometers (step (c)). Thep-type diffused layer becomes the isolation layer 5.

Thereafter, although not particularly illustrated, after a top surfacestructure is formed, the semiconductor wafer 1 is made thinner bygrinding its bottom surface until the bottom surface 6 reaches theisolation layer 5. On the ground bottom surface 6, a bottom surfacestructure made up of a p⁺ collector region and a collector electrode isformed. Subsequently, the semiconductor wafer 1 is cut at a scribingline positioned at the center of the isolation layer 5 to form an IGBTchip.

FIG. 11 is a cross-sectional view showing the principal part of therelated reverse-blocking IGBT whose isolation layer 5 is formedaccording to the manufacturing steps shown in FIG. 10. Reference numeral8 denotes a p⁺ well region, 10 denotes a p voltage withstanding region,9 denotes a gate insulator film, 12 denotes a field oxide film, 7denotes a p⁺ collector region and 11 denotes a dicing face. Illustrationof an emitter region selectively formed on the surface in the p wellregion 8, a gate electrode on the gate insulator film 9, an interlayerinsulator film further covering the gate electrode, an emitter electrodeon the interlayer insulator film, a field plate formed on the fieldoxide film 12, and a collector electrode covering the collector region 7are not illustrated.

The isolation layer is formed in another way in a relatedreverse-blocking IGBT by providing a trench and forming a diffusionlayer on the side wall of the trench as shown in FIGS. 12A-12C. Here, ona semiconductor wafer 14, an etching mask is first formed with a thickoxide film 13 having a thickness of several micrometers (FIG. 12A).Next, a trench 15 having a depth of the order of several hundredsmicrometers is formed by dry etching (FIG. 12B). Then, impurities, suchas dopant gas 16, which can be diborane (B₂H₆), is introduced into theside wall of the trench 15 via vapor phase diffusion to form anisolation layer 17 of a p-type diffused layer (FIG. 12C). In this case,after the trench 15 is filled back with a reinforcing material of anepitaxial silicon film or a polysilicon film, dicing is carried outalong a scribing line, by which an IGBT chip is cut from thesemiconductor wafer 14.

Such a method of providing the trench 15 and forming the isolation layer17 on the side wall of the trench 15 is disclosed in JP-A-2-22869,JP-A-2001-185727, and JP-A-2002-76017. JP-A-2-22869 discloses forming atrench from the top surface of a device to a bottom side junction so asto surround an active layer, and then forming a diffusion layer on theside wall of the trench to form an isolation layer with an end of thebottom side junction of the device made extending to the top surface ofthe device. JP-A-2001-185727 and JP-A-2002-76017 disclose, likeJP-A-2-22869, forming a trench from the top surface of the device to abottom junction and then forming a diffused layer on the side wall ofthe trench to thereby form a device having a reverse-blockingcapability.

In the method of forming the isolation layer in the reverse-blockingIGBT shown in FIGS. 10 and 11, a long time diffusion treatment in a hightemperature environment is necessary for diffusing boron from the boronsource 4 (a liquid diffusion source of boron) to coat on the surface toform the isolation layer 5 with a diffusion depth of the order ofseveral hundreds micrometers. This, however, can fatigue quartzfixtures, forming a diffusion furnace, such as a quartz board, a quartztube, and a quartz nozzle, and contaminate by foreign materials from aheater and strength reduction due to devitrification of the quartzfixtures.

Moreover, in forming the isolation layer 5 by the coating and diffusionmethod, it becomes necessary to form a masking oxide film (the oxidefilm 2). The masking oxide film needs to be thick and high quality towithstand long time boron diffusion. A silicon oxide film with highresistance of mask, that is, with a high quality, can be formed by athermal oxidation method. However, in order that no boron atomspenetrate through the masking oxide film in the high temperature andlong time (at 1300° C. for 200 hours, for example) diffusion processingof the isolation layer 5 with boron, it is necessary to form a thermaloxide film with a film thickness of about 2.5 μm. For forming such athermal oxide film with the film thickness of about 2.5 μm, an oxidationtime required at an oxidation temperature of 1150° C., for example, isabout 200 hours in the dry atmosphere of oxygen by which a high qualityoxide film can be obtained.

Even with wet or pyrogenic oxidation, which is known to shortenoxidation time in comparison with the dry oxidation, albeit with slightinferiority in quality of an obtained oxidized film, a long oxidationtime of about 15 hours is still necessary. Furthermore, in the aboveoxidation processing, a large amount of oxygen is introduced into asilicon wafer. This introduces crystal defects, such as oxygenprecipitates and oxidation-induced stacking faults (OSF), and producesoxygen donors to thereby cause adverse effects, such as characteristicsdeterioration and reliability degradation of the device.

Furthermore, also in the step of diffusing boron carried out after theboron source 4 has been coated, the above high temperature and long timediffusion processing is usually carried out under an oxidizingatmosphere. This causes oxygen atoms to be introduced into crystallattices in the wafer as interstitial oxygen atoms. Thus, also in thediffusion step, crystal defects, such as oxygen precipitates, oxygendonor production, OSF, and slip dislocations are introduced. It is knownthat a leak current is increased in a p-n junction formed in a waferwith such crystal defects and a breakdown voltage and reliability aresignificantly degraded in an insulator film formed on the wafer bythermal oxidation. Moreover, oxygen atoms taken in during diffusionprocessing are made to become donors by another heat treatment to causean adverse effect of lowering the breakdown voltage.

In the method of forming the isolation layer shown in FIGS. 10 and 11,approximately isotropic diffusion of boron progresses toward a siliconbulk from the opening of the masking oxide film. Thus, the borondiffusion up to 200 μm in the depth direction causes the boron to beinevitably diffused also in the lateral direction on the order of 180μm. This causes an adverse effect on reduction in a device pitch and achip size.

In the manufacturing method shown in FIGS. 12A-12C, the trench 15 isformed by dry etching and boron is introduced into the side wall of theformed trench 15 to form the isolation layer 17. Thereafter, the trench15 is filled with the reinforcing material such as an insulator film ora semiconductor film. Since a trench with a high aspect ratio can beformed, the forming method shown in FIGS. 12A-12C is more advantageousfor reducing a device pitch as compared with the forming method shown inFIG. 10. However, the processing time required for etching to a depth ofthe order of 200 μm is on the order of as long as 100 minutes per onewafer when a typical dry etching device is used. This brings adverseeffects such as an increase in a lead time and the number ofmaintenance. Moreover, when a deep trench is formed by dry etching witha silicon oxide (SiO₂) film used as a mask, a thick silicon oxide filmwith a thickness of several micrometers is necessary because the etchingselectivity is on the order of 50. The thick silicon oxide film causesadverse effects such as increase in a cost and reduction in a rate ofacceptable products due to introduction of process induced crystaldefects, such as OSFs and oxygen precipitates.

Further, in the process of forming an isolation layer in which a deeptrench with a high aspect ratio formed by dry etching, there is aproblem in that residues, such as a chemical residue 18 and aphoto-resist residue 19 are left in the trench as shown in FIG. 12D tocause adverse effects, such as reduction in yield and reduction inreliability. When a dopant such as phosphorus or boron is introducedinto the side wall of a trench, the vertically provided side wall of thetrench causes the dopant introduction to be usually carried out byimplanting dopant ions with the wafer inclined.

Introduction of a dopant into the side wall of the trench having a highaspect ratio is carried out by ion implantation incident at a slightangle of inclination (ion implantation from the top surface of a waferonto a side wall of a trench having a large angle of inclination to thebottom surface of the wafer). This causes, as shown in FIGS. 13A and13B, and 14, each being a schematic diagram for explaining problems inion implantation incident at a slight angle of inclination, adverseeffects, such as reduction in an effective dose (and an accompaniedincrease in an implantation time), a decrease in an effective projectedrange, a dose loss due to presence of a screen oxide film 14, andreduction in implantation uniformity. Moreover, in a side wall 15 of atrench, a dose and a depth of implanted dopant ions 13 are reducedcompared with those to the top surface of the wafer as a principalsurface. Furthermore, implanted dopant ions 13 are sometimes absorbed inthe screen oxide film 14, reflected, and re-emitted to cause a doseloss. In addition, as shown in FIG. 6A as a characteristic diagramshowing a relationship between a angle of inclination and a relative ionimplantation time, for a trench formed by ordinary dry etching to have aside wall with a large angle of inclination of 80° or more to the bottomsurface, a very long implantation time is required when an ion beam ismade incident vertically to a wafer. Therefore, as a measure taken foreffectively introducing an impurity into a trench having a high aspectratio, vapor phase diffusion is used instead of implanting dopant ionsinto a wafer. In the vapor phase diffusion, a wafer is exposed to agasified atmosphere of a dopant, such as PH₃ (phosphine) or B₂H₆(diborane). The vapor phase diffusion, however, is inferior in finecontrollability of dose as compared with ion implantation. Moreover,doses of dopant ions that can be introduced are often limited by theirsolubility limits.

Moreover, when a trench having a high aspect ratio is filled with aninsulator film, a space referred to as a void is produced in the trenchto cause a problem, such as reduction in reliability. When a deep trenchis formed by dry etching, the surface of a wafer is exposed to a plasmaatmosphere for a long time. This causes a plasma damage of the wafer,which degrades the device characteristics. In particular, a gatestructure in an IGBT is susceptible to plasma damage. Therefore, thetrench formation by dry etching is limited to a formation step carriedout before a gate structure formation step. Furthermore, even after thegate structure has been formed, there still remains many semiconductormanufacturing process steps that must be undergone, such as a formationstep of an emitter structure and a formation step of a passivationlayer. When such processing steps are carried out with the formed trenchleft exposed, residues of resists and chemicals collected therein cancause the product to malfunction. Hence, the trench must be filled witha semiconductor film or an insulator film, which increases themanufacturing cost. Furthermore, when a trench formed by dry etchingwith a high aspect ratio is filled with an insulator film or asemiconductor film, a space referred to as a void is produced in thetrench to sometimes cause a reliability problem.

Moreover, in forming an isolation layer by coating and diffusion, asection where the collector diffused layer and the diffused layer of theisolation layer connect with each other forms a sharp angle at the edgeof the bottom of the chip. This can degrade the breakdown voltage due toelectric field concentration. Furthermore, when a V-shaped trench isformed by etching, carried out from a first principal surface (topsurface) side on which a MOS gate structure is formed, a device pitch isincreased. In addition, the section where the collector diffused layerand the diffused layer of the isolation layer connect with each otherforms a sharp angle at the edge of the bottom of the chip. This can alsodegrade the breakdown voltage due to electric field concentration.Moreover, a vertically formed trench tends to cause a laser beam to beincident parallel to a trench side wall to make it more difficult toactivate the dopant impurities at the side wall.

Accordingly, there still remains a need for improving the manufacturingprocess for a semiconductor device, where dopant ions implanted into aside wall of a trench can be more readily activated. There still remainsa need for a semiconductor device having high reliability, a smalldevice pitch and a small chip size. Moreover, there still remains a needfor improving the manufacturing a semiconductor device, where anisolation layer can be formed without carrying out high temperature andlong time diffusion processing and long time oxidation processing. Thepresent invention addresses these needs.

SUMMARY OF THE INVENTION

The present invention relates to a power semiconductor device, such as apower conversion device, and a manufacturing method thereof. Inparticular, the present invention relates to a method of forming anisolation layer in a bidirectional device or a reverse-blocking devicehaving bidirectional voltage withstanding characteristics, or toformation of trenches of a sensor, an actuator, and a solar cell towhich an MEMS (Micro-Electro-Mechanical-System) technology is applied.

One aspect of the present invention is a method of manufacturing asemiconductor device. Here, the method can include forming a trenchhaving a substantially V-shaped or trapezoidal-shape cross section, in asecond principal surface of a first conductivity type semiconductorsubstrate having the first principal surface and a second principalsurface, introducing a second conductivity type impurity into a sidewall of the trench, and activating the impurity by laser irradiation.The side wall of the trench is formed to have an angle of inclination of70° or less relative to the second principal surface. More specifically,the side wall of the trench is formed so as to have an angle ofinclination between 30° and 70° relative to the first principal surface.

Another aspect of the present invention is a semiconductor device havinga substrate with a first principle surface and a second principlesurface. A trench is formed in one of the first principle surface or thesecond principle surface. A doped semiconductor layer is on the trench.The trench has a substantially V-shaped or trapezoidal-shaped crosssection and has side walls that extend from the one principle surface atan incline. The angle of inclination of the side walls relative to theother principle surface is at least 30° but not greater than 70°.

Another aspect of the present invention is a semiconductor device havinga second conductivity type base region selectively provided in a surfaceregion on a first principal surface of a first conductivity typesemiconductor substrate, a first conductivity type emitter regionselectively provided in a surface region of the base region, and a MOSgate structure. The MOS gate structure includes a gate insulator filmprovided on a surface of a section of the base region and the gateelectrode provided on the gate insulating film. The section ispositioned between the semiconductor substrate and the emitter region. Agate electrode can be provided on the gate insulator film. An emitterelectrode can contact with the emitter region and the base region. Asecond conductivity type collector layer can be provided on a surface ofa second principal surface of the semiconductor substrate. A collectorelectrode can be in contact with the collector layer. A secondconductivity type isolation layer can surround the MOS gate structure,extend to the first principal surface from the second principal surfacewhile being inclined to the first principal surface, and be coupled tothe collector layer. The isolation layer can be covered with thecollector electrode.

Another aspect of the present invention is a method of manufacturing theabove semiconductor device. The method can include forming the MOS gatestructure on the first principal surface of the first conductivity typesemiconductor substrate, covering the second principal surface of thefirst conductivity type semiconductor substrate with a mask havingopenings of a desired pattern along the <110> direction, forming atrench having a V-shaped or trapezoidal-shape cross section in thesemiconductor substrate from the second principal surface side, by wetanisotropic etching with an alkaline solution, sections of the secondprincipal surface of the semiconductor substrate not covered by themask, and forming the second conductivity type isolation layer and adiffused layer on the second principal surface by introducing a secondconductivity type impurity into a side wall of the trench and the secondprincipal surface.

Another aspect of the present invention is a method of manufacturing asemiconductor device. The method can include forming a plurality ofdiffused layers, a first main electrode and a control electrode on afirst principal surface side of a semiconductor wafer, thinning thesemiconductor wafer by grinding a second principal surface of thesemiconductor wafer, affixing the first principal surface side to asupporting substrate with an adhesive layer, forming a trench thatextends to the diffused layer on the first principal surface side from asecond principal surface side of the thinned semiconductor wafer,forming an isolation layer on an entire area of a side wall of thetrench and a diffused layer on the second principal surface side on thesecond principal surface, each of the isolation layer and the diffusedlayer having the same conductivity type as the diffused layer on thefirst principal surface side, forming a second main electrode in contactwith the diffused layer on the second principal surface side and theisolation layer, and removing the adhesive layer from the thinnedsemiconductor wafer to provide the semiconductor wafer as asemiconductor chip.

The crystal face of each of the first principal surface and the secondprincipal surface can be a {100} plane, and the crystal faces of theside wall of the trench and the isolation layer can be a {111} plane.The laser irradiation can be carried out with a laser beam made incidentonto the second principal surface of the semiconductor substrate at anangle of incidence up to ±30° to the normal to the second principalsurface. The laser irradiation can be carried out with a focal pointposition of a laser beam made to come on the side wall of the trench sothat no molten crystal mark or work mark appears on the first principalsurface of the semiconductor substrate.

The isolation layer can be an impurity layer formed by introducing asecond conductivity type impurity into the side wall of the {111} planeof a trench formed in the semiconductor substrate from the secondprincipal surface, and the second conductivity type isolation layer canhave an angle of inclination of 125.3° on the second principal surfaceside.

The trench can be formed so as to be inclined at an angle of 125.3°relative to the second principal surface, and the second conductivitytype impurity can be introduced into the side wall by ion implantation.The collector electrode can cover the second conductivity type isolationlayer.

The mask can be made of at least one of a silicon oxide film, a siliconnitride film, an SOG film, and an alkali resistant resist film. Thesilicon oxide film and the silicon nitride film can be formed by plasmaor atmospheric pressure CVD, and the SOG film can be formed by spincoating.

The trench can be formed by wet anisotropic etching. The trench canextend to the first principal surface. The etching of the trench can bestopped at a silicon oxide film or a silicon nitride film formed on thefirst principal surface. The isolation layer and the diffused layer onthe second principal surface side can be formed by ion implantation andlow temperature annealing at 550° C. or below. The isolation layer andthe diffused layer on the second principal surface side can be formed byion implantation and laser annealing.

The irradiation energy density of the laser beam can be 0.25 to 5.0Joule/cm² from a YLF2ω laser or a YAG2ω laser, with which the entiresurface of the second principal surface is irradiated with a delay timefor second irradiation given as 0 to 5000.

When the crystal face of the side wall of the trench is a {111} plane,the crystal face is inclined at 35.3° to the plane perpendicular to thesecond principal surface. Hence, a laser beam made incident onto thesecond principal surface of the semiconductor substrate at an angle ofincidence up to 30° to the normal to the second principal surfaceenables laser irradiation onto both of the side walls of the trench.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a plurality of trenches, each witha side wall having a different angle of inclination, formed on a surfaceof a semiconductor wafer according to the present invention.

FIG. 2A is a characteristic diagram showing measured impurityconcentration profiles by angle of inclination of the trench.

FIG. 2B is a characteristic diagram showing impurity concentrationprofiles measured at several positions on the inclined side wall of atrench away from the focal point of a laser beam set at the bottom ofthe trench.

FIG. 2C is a characteristic diagram showing measured impurityconcentration profiles in the depth direction measured by irradiationenergy density.

FIG. 2D is a characteristic diagram showing results of carrierconcentration profile measurements respectively carried out about theflat plane section and the trench side wall section of a wafer with aV-shaped trench formed by wet anisotropic etching with an alkalinesolution.

FIG. 3 is schematically illustrates the manufacturing steps for formingthe reverse-blocking IGBT according to the invention.

FIG. 4A is a schematic cross-sectional view showing a width of anisolation layer provided by coating and diffusion and a device pitch ina wafer in the related art.

FIG. 4B is a schematic cross-sectional view showing a width of anisolation layer and a device pitch when forming a V-shaped trench fromthe top surface of a wafer.

FIG. 4C is a schematic cross-sectional view showing a width of anisolation layer and a device pitch when forming a V-shaped trench fromthe bottom surface of a wafer according to the present invention.

FIG. 4D is a partial cross-sectional view illustrating effect ofvariation in the thickness of a wafer on spacing between chips.

FIG. 5A is an enlarged cross-sectional view showing a section of anisolation layer in one embodiment of a reverse-blocking IGBT accordingto the present invention.

FIG. 5B is a plan view showing a pattern of trenches for nine chipsformed by wet anisotropic etching of silicon with an alkaline solution.

FIG. 5C is a cross-sectional view of FIG. 5B.

FIG. 6A is a characteristic diagram showing a relationship between anangle of inclination of the side wall of a trench and a relative ionimplantation time.

FIG. 6B is a characteristic diagram showing a relationship between theangle of inclination of the side wall of the trench and a rate ofcracking of chips.

FIGS. 7A and 7B illustrate a wafer in a plan view and a cross-sectionalview where the spacing between chips is larger.

FIGS. 7C and 7D illustrate a wafer in a plan view and a cross-sectionalview where the spacing between chips is smaller.

FIG. 8A is a schematic cross-sectional view showing a reverse-blockingIGBT chip according to related art separated at an isolation layerformed by coating and diffusion to have an edge of the bottom surfaceforming a right angle.

FIG. 8B is a schematic cross-sectional view showing a reverse-blockingIGBT chip according to related art separated at a V-shaped trench formedfrom the top surface to have an edge of the bottom surface forming anacute angle.

FIG. 8C is a schematic cross-sectional view showing a reverse-blockingIGBT chip according to the invention separated at a V-shaped trenchformed from the bottom surface to have an edge of the bottom surfaceforming an obtuse angle.

FIG. 9 is a schematic perspective view showing wet anisotropic etchingof silicon with an alkaline solution according to the present invention.

FIG. 10 schematically illustrates the manufacturing steps for forming aprincipal part of a related reverse-blocking IGBT.

FIG. 11 is a cross-sectional view showing a principal part of therelated reverse-blocking IGBT whose isolation layer is formed accordingto the manufacturing steps shown in FIG. 10.

FIG. 12A is a cross-sectional view showing a principal part of anotherrelated reverse-blocking IGBT at the manufacturing step in anotherrelated manufacturing method at which step an oxide film as an etchingmask is formed on a semiconductor wafer.

FIG. 12B is a cross-sectional view showing the principal part of therelated reverse-blocking IGBT at the manufacturing step next to the stepshown in FIG. 12A at which a trench is formed by dry etching.

FIG. 12C is a cross-sectional view showing the principal part of therelated reverse-blocking IGBT at the manufacturing step next to the stepshown in FIG. 12B at which an isolation layer is formed on a side wallof the trench by vapor phase diffusion.

FIG. 12D is a cross-sectional view showing the principal part of therelated reverse-blocking IGBT at the manufacturing step shown in FIG.12B with a chemical residue and a photo-resist residue being left in thetrench formed by dry etching.

FIGS. 13A and 13B schematically illustrate a perspective view and across-sectional view for explaining problems in ion implantationincident at a slight angle of inclination.

FIG. 14 schematically illustrates a cross-sectional view for explainingproblems in a side wall of a trench in the ion implantation incident ata slight angle of inclination.

DETAILED DESCRIPTION

In the following, preferred embodiments of a method of manufacturing thedevice according to the invention will be explained in detail withreference to the attached drawings. Here, the explanations will be madein relation to examples in each of which the invention is applied to areverse-blocking IGBT. In the following explanations and the attacheddrawings, a leading character “n” or “p” attached to names of layers andregions means that majority carriers in the layers and the regions areelectrons or holes, respectively. Moreover, a sign “⁺” attached to theleading character “n” or “p” means that the layer and the region have acomparatively high impurity concentration, and a sign “⁻” attached tothe leading character “n” or “p” means that the layer and the regionhave a comparatively low impurity concentration. Furthermore,arrangements denoted with the same reference numerals and signs aresimilar, so that redundant explanations will be omitted.

In the manufacturing process of the reverse-blocking IGBT, a gate andemitter structure of IGBT on the top surface (first principal surface)side of a wafer is first formed and the bottom surface (second principalsurface) side of the wafer is thinned down to a specified thickness.Thereafter, for forming a collector region (an isolation layer)connecting the top surface and the bottom surface, an etching mask isformed on the bottom surface with its pattern made to correspond to thepattern on the top surface by using a double-sided mask aligner. Then, aV-shaped or trapezoid-shaped trench provided on the periphery of a chipis formed by wet anisotropic etching of silicon with an alkalinesolution carried out from the bottom surface of the wafer.

Into the side wall of the trench formed by the etching from the bottomsurface side of the wafer, a dopant such as boron is introduced by ionimplantation. The introduced dopant is then activated by furnaceannealing up to 550° C. or laser annealing so that the IGBT structure onthe top surface, by which an isolation layer is formed, is not thermallydamaged. Thereafter, over the entire bottom surface of the wafer, metalfilms for a collector electrode is deposited.

When the trench (V-shaped trench) is formed by wet anisotropic etchingwith an alkaline solution, the side wall of the trench is formed to havea small angle of inclination relative to the top surface of the wafer ora large angle of inclination relative to the bottom surface of thewafer. This also enables a collector diffused layer on the bottomsurface of the wafer and an isolation diffused layer on the side wall ofthe trench to be formed together. After the diffused layers are formed,the metal collector electrode is formed over the entire bottom surfaceby sputtering or evaporation. This deposits electrode metals on the sidewall of the trench. The deposited metals also can serve as a protectionfilm of the isolation diffused layer. Thus, the isolation layer can beformed with no or reduced defects caused by flaws and defects fromcontaminants.

On the bottom surface of the wafer, an etching mask is formed with itspattern aligned with the pattern on the top surface by using adouble-sided mask aligner so that the bottom face of the trench, formedfrom the bottom surface of the wafer, surrounds an active region on thetop surface after etching. For the material of the etching mask, asilicon oxide film or a silicon nitride film can be used, which can beprovided by plasma CVD or atmospheric pressure CVD. However, analkali-resistant resist or an alkali-resistant photosensitive resin alsocan be conveniently used.

Referring to FIG. 9, which is a schematic perspective view showing wetanisotropic etching of a silicon wafer 28 with an alkaline solution, inthe etching process, wet anisotropic etching of silicon with an alkalinesolution can be used. For wet anisotropic etching, solutions forsilicon, aqueous solutions of alkalis such as KOH (potassium hydroxide),hydrazine, ethylenediamine, ammonia, and TMAH (tetramethylammoniumhydroxide) have long been known and widely used. Etching of siliconusing these alkaline solutions has anisotropy (dependence on directionof crystal plane of the etching rate of silicon). Specifically, etchingrates for a (111) plane, a (110) plane and a (100) plane when a KOHsolution is used are in the ratios 1:600:400, by which etching actuallystops on a crystal face {111} equivalent to the (111) plane. Thus, it isknown that the etching carried out with an etching mask, having openingsalong the <110> direction and the direction perpendicular to the <110>direction, formed beforehand on the surface of a wafer as the (100)plane allows a V-shaped trench, a trapezoidal-shaped trench, or apyramid-shaped trench structure to be formed. Moreover, it is knownthat, by adjusting the width of the opening of the mask 20 or an etchingtime, the V-shaped trench, a trapezoid-shaped trench, or thepyramid-shaped trench can be formed with an arbitrary depth and anarbitrary size. When the etching is stopped halfway, a trench can beformed with a cross section having an inverted-trapezoidal shape, suchas shown in FIG. 9. When the etching is further progressed, the {111}plane of the side wall of the trench is increased. While, the (100)plane at the bottom of the trench is decreased to finally disappear withthe {111} planes on both sides intersecting, at which further etching issubstantially self-stopped. This causes no variation in the depth of theV-shaped trench even though the etching time varies. Thus, once theopening width of the etching mask 20 is determined, the depth of theV-shaped trench is determined. Specifically, the depth of the V-shapedtrench is calculated with ½ of the opening width multiplied by tan54.7°. The mask 20 can be formed of a silicon oxide or nitride film.

For etching solutions, an aqueous solution of TMAH (tetramethylammoniumhydroxide) and an aqueous solution of ammonia are conveniently usedbecause there is no problem of contamination by metallic ions, and anetching rate for silicon oxide film used as a mask is very small. In themethod according to the invention, however, the trench is formed byanisotropic etching, at a later stage of the process, after thepassivation film is already formed over the IGBT structure to minimizethe inclusion contaminants. An aqueous solution of KOH can be used toetch, which shortens the etching time.

Furthermore, a passivation layer formed beforehand in an isolation layerregion on the top surface with a silicon oxide film or a silicon nitridefilm prevents the trench, formed from the bottom surface of the wafer bywet anisotropic etching with an alkaline solution, from being furtheretched when the bottom of the trench reaches the passivation layer onthe top surface of the wafer. Thus, the passivation layer is effectivefor naturally stopping the etching.

After the V-shaped or the trapezoid-shaped trench is formed by the wetanisotropic etching of silicon with an alkaline solution, boron ions asdopant ions are introduced into the side wall of the trench by ionimplantation. Then, heat treatment is carried out to activate the boronions, by which a p⁺ diffused layer (the isolation layer) is formed. Theboron ions can be implanted into the side wall of the trench with thewafer inclined like in an ordinary ion implantation. However, since theangle of inclination of the side wall is 54.7° relative to the topsurface of the wafer or supplementary angle of 125.3° relative to thebottom surface, the implantation can be carried out without angling thewafer to make ions incident perpendicularly to the wafer. Furthermore,the angle of inclination of 54.7°/125.3° causes no extreme difference ineffective implanting dose and implanted depth between the bottom surfaceof the wafer perpendicular to the incident ions and the side wall of thetrench inclined to the incident ions. Therefore, it is also possible tocombine boron ion implantation for forming the collector diffused layeron the bottom surface of the wafer and boron ion implantation forforming the isolation layer on the side wall of the trench together in asingle implantation step. This reduces the number of process steps,which is effective in cost reduction and enhancement of a rate ofacceptable products.

FIGS. 5A-5C are cross-sectional views for explaining an example of areverse-blocking IGBT according to the present invention. FIG. 5A is anenlarged cross-sectional view showing a section of an isolation layer.As is shown in FIG. 5A, on a top surface (upper side of FIG. 5A) of ann⁻ silicon semiconductor substrate 1 (semiconductor wafer with highresistivity), a plurality of p⁺ base regions 8 are selectively formed.On a bottom surface (lower side of FIG. 5A) of the substrate 1, a p⁺collector layer 7 is formed. The region between the p⁺ base regions 8and the p⁺ collector layer 7 in the direction of the thickness of thesubstrate is originally the n⁻ single crystal silicon semiconductorsubstrate 1, which becomes an n base region. Although not particularlylimited, the thickness of the n⁻ silicon semiconductor substrate 1, thatis, the dimension from the first principal surface to the secondprincipal surface, can be 200 μm, for example.

In an active region 24, part of which is shown by an arrow, an n⁺emitter region 49 is selectively formed in each p⁺ base region 8. On theoutside of the active region 24, a voltage withstanding structure isformed as one type of end structures on the surface of a planar p-njunction to ensure a forward-blocking breakdown voltage of the IGBT. Thevoltage withstanding structure is positioned on the outside of theactive region 24 on the top surface. Moreover, the voltage withstandingstructure is formed by combining a plurality of stages each including aguard ring of a p⁺ semiconductor region (not shown), an oxide film 12and a field plate of a metal film which are formed in ring-like on thetop surface layer of the n⁻ silicon semiconductor substrate 1.

On the surface of the p⁺ base region 8 positioned between the n⁺ emitterregion 49 and the n base region (the n⁻ silicon semiconductor substrate1), on the surface of the n base region between the p⁺ base regions 8adjacent to each other, and on the surface of the adjacent p⁺ baseregion 8 positioned between the n base region and one of the n⁺ emitterregions 49 in the adjacent p⁺ base region 8, a gate electrode (notshown) is formed over these surfaces with a gate insulator film 9 heldbetween the gate electrode and the surfaces. The surface of the n⁺emitter region 49 is covered with an emitter electrode (not shown). Thesurface of the p⁺ collector layer 7 is covered with a collectorelectrode 29. Between the emitter electrode and the gate electrode, aninterlayer insulator film is provided.

On the outside of the voltage withstanding structure, a p⁺ isolationlayer 25 is formed. The p⁺ isolation layer 25 is formed along a sidewall of a trench 26 formed from the bottom surface. The side wall of thetrench 26 is inclined at an angle of about 54.7° with respect to the topsurface or 125.3° with respect to the bottom surface. Therefore, the p⁺isolation layer 25 is inclined at an angle of about 54.7°/125.3° with astrip-like cross sectional shape.

With the p⁺ isolation layer 25 thus provided, a depletion layer,spreading before and behind the p-n junction when a reverse bias isapplied, can be prevented from spreading over the dicing face as an edgeof a chip of the IGBT and a damage region around the dicing face, bywhich a sufficient reverse breakdown voltage can be obtained. Referencenumeral 27 denotes a passivation layer covering the emitter electrode.

In the example, for forming the p⁺ isolation layer 25, the trench 26 isformed in the n⁻ silicon semiconductor substrate 1 by carrying out wetanisotropic etching with an alkaline solution. The trench 26 has aV-shaped or trapezoid-shaped cross sectional shape with the side wallinclined at an angle of about 54.7°/125.3°.

FIGS. 5B and 5C illustrate a plan view and a cross-sectional view,respectively, each showing a pattern of trenches for nine chips formedby wet anisotropic etching of silicon with an alkaline solution. In FIG.5B, a (100) plane is in the active region 24 of the device, and a (111)plane, a (11 1) plane, a (1 1 1) plane and a (1 11) plane form the sidewall of the trench 26. FIG. 5C shows a supporting substrate 19.

For wet anisotropic etching solutions for silicon, aqueous solutions ofalkalis such as potassium hydroxide, hydrazine, ethylenediamine, ammoniaand TMAH (tetramethylammonium hydroxide) are used. Etching of siliconusing such solutions are carried out with characteristics that etchingrates differ depending on directions of crystal planes of silicon, i.e.,with anisotropy. Specifically, in using a solution of potassiumhydroxide, for example, the etching rates for a (110) plane and the(100) plane are 600 times and 400 times, respectively, that for the(111) plane. Namely, etching actually stops on a crystal planeequivalent to the (111) plane.

Thus, it is known that the etching, carried out with an etching maskhaving an opening along the direction of a {110} plane and formedbeforehand on a wafer whose surface is the (100) plane, allows aV-shaped trench, a trapezoidal-shaped trench or a pyramid-shaped trenchstructure to be formed. Moreover, it is known that, by adjusting thewidth of the opening of the etching mask or an etching time, theV-shaped trench, a trapezoid-shaped trench or the pyramid-shaped pit canbe formed with an arbitrary depth and an arbitrary size.

When an opening width of an etching mask 20 is narrow, with the (100)plane at the bottom of the trench 26 disappeared with the {111} planeson both of the opposite sides intersecting at an angle of about 70.6°,further etching is substantially stopped. This causes no variation inthe depth of the V-shaped trench 23 even though the etching time varies.When the opening width of the etching mask 20 is wide, the trench 26 canbe formed with a cross section having a substantiallyinverted-trapezoidal shape. In this case, an angle formed by each of the(111) plane, the (1 1 1) plane and the (1 11) plane to become the sidewall of the trench 26 and the passivation layer 27 exposed by etching isapproximately 125.3°. Therefore, photo-resist residues and chemicalresidues are more easily removed than in the case of the V-shapedtrench.

The depth of the V-shaped trench 26 has a value for which ½ of theopening width of the etching mask is multiplied by tan 54.7°. Forforming the V-shaped trench 26 so as to have a desired depth, it isnecessary only for the opening width of the etching mask to have a valuefor which the depth of the trench 26 is multiplied by 2/tan 54.7°. Forexample, when the trench is to be provided with a depth of 200 μm, theopening width of the etching mask can be provided as 283 μm, which isadvantageous for reduction of a device pitch. In this case, however, thebottom of the trench 26 forms a somewhat sharp acute angle of about70.6°. Thus, it is preferable to round the corner of the bottom bycarrying out processing such as hydrogen annealing processing, cornerrounding oxidation processing, or CDE (Chemical Dry Etching).

In wet anisotropic etching of silicon with an alkaline solution, a largeetching mask selectivity allows the masking oxide film to be providedvery thin. Even with a silicon oxide film formed by CVD (Chemical VaporDeposition), a sufficient etching mask selectivity can be obtained,although such a silicon oxide film is a little inferior to a thermaloxide film in film quality (resistance of mask). Thus, a film such as aTEOS (Tetraethylorthosilicate) film formed by reduced pressure CVD orplasma-assisted CVD can be also provided as the masking oxide film.

Since the angle of inclination of the side wall of the trench 26 isrelatively large, about 125.3° relative to the bottom surface side,impurity ions can be implanted into the side wall of the trench 26without inclining the silicon wafer. That is, the ion implantation intothe silicon wafer can be carried out at a tilt angle of 0°. In thiscase, impurity ions can be simultaneously implanted into the four sidewalls of the (111) plane, the (11 1) plane, the (1 1 1) plane and the (111) plane by one time ion implantation. Hence, the ion implantation canbe easily carried out. Here, however, like in ion implantation into anordinary trench side wall, impurity ions can be separately implantedinto each of the four side walls of the (111) plane, the (11 1) plane,the (1 1 1) plane and the (1 11) plane by inclining the silicon wafer

Since there is no necessity to thicken the etching mask when the trench26 is formed, the masking oxide film to be the etching mask can beformed by carrying out thermal oxidation at a temperature lower thanthat in a related method for a time shorter than that in the relatedmethod. This can reduce a problem of increasing the lead time and aproblem of causing crystal defects due to oxygen introduction atoxidation. Moreover, in wet anisotropic etching with an alkalinesolution, with an etching rate determined very high, etching can becarried out in a batch process. This is very largely effective in leadtime reduction and in cost reduction.

In the wet anisotropic etching with an alkaline solution, an etchingtemperature is as low as 200° C. or below. This causes very smallthermal damage that exerts no influence on the dopant profile of theactive region 24. Moreover, even though a metal having a comparativelylow melting point such as aluminum and a material having poor heatresistance are formed on the silicon wafer before the trench 26 isformed by the wet anisotropic etching, the carried out etching exerts noinfluence on the metal and the material. Moreover, there is no variationin the angle of inclination of the side wall of the trench 26, so thatvariations in a dose and in ranges of implanted ions at ion implantationbecome far small.

Next, an explanation will be made about affixing the passivation layerof the thin semiconductor wafer onto a supporting substrate 19, whichcan be formed of a material such as quartz glass (a glass wafer) with adouble-sided adhesive tape put therebetween. The thin semiconductorwafer and the supporting substrate are combined with a double-sidedadhesive tape put between them and pressure is applied on both the thinsemiconductor wafer side and the supporting substrate side, or a rolleris applied on both of them so that no air-bubbles are contained betweenaffixed faces. Thus, the thin semiconductor wafer and the supportingsubstrate are affixed together.

The double-sided adhesive tape having a structure in which a thermalfoaming tape, which is removable by heating and a UV (Ultraviolet) tape,which is removable by irradiation with ultraviolet light, for example,are affixed with a PET (Polyethylene Terephthalate) film put between. Tothe thermal foaming tape, the passivation layer of the thinsemiconductor wafer is affixed, while, to the UV tape, the supportingsubstrate is affixed. Although not particularly limited, each of thethermal foaming tape and the UV tape can have a thickness of 50 μm andthe PET film can have a thickness of 100 μm, for example. Moreover, thesupporting substrate can have a thickness of 600 μm, for example.

Next, between adjacent IGBT chip forming areas in the thin semiconductorwafer, a trench to be a scribing line region is formed by wetanisotropic etching. The bottom of the trench is made to reach the topsurface. In this state, even though the trench is formed, the thinsemiconductor wafer, being secured to the supporting substrate with thedouble-sided adhesive tape put between, is not separated intosemiconductor chips. The side wall of the trench becomes an edge of anIGBT chip.

Requirements of an etching solution for forming the trench are 3 to 20%in concentration and 50 to 90° C. in temperature in a TMAH(tetramethylammonium hydroxide) aqueous solution. Moreover, requirementscan be 1 to 20% in concentration and 50 to 90° C. in temperature in anNH₄OH (ammonia) aqueous solution, and can be 10 to 60% in concentrationand 50 to 90° C. in temperature in a KOH (potassium hydroxide) aqueoussolution.

The trench thus formed has a shape with its side wall being the {111}plane as explained before because the surface of the thin semiconductorwafer is the {100} plane. The flatness of the {111} plane etched by thewet anisotropic etching is of the order of 1 nm Ra with which the planebecomes very smooth. In the case in which an opening width of a mask isnarrow and etching is naturally stopped to form a trench with a V-shapedcross section, the trench is provided so that the bottom of the V-shapereaches the top surface. While, in the case in which the opening widthof the mask is wide and etching is stopped halfway to form a trench withan inverted-trapezoid-shaped cross section, the trench is provided sothat the bottom of the inverted-trapezoid-shape reaches the top surface.

Thereafter, ion implantation with boron ions is carried out from thebottom surface side of the thin semiconductor wafer onto the side wallof the trench. Then, low temperature annealing is carried out toactivate the implanted boron ions, by which an isolation layer isformed. In forming the isolation layer, like in the ion implantationinto a vertical trench side wall, the ion implantation can be carriedout by dividing the implantation process into four steps each beingcarried out for the side wall in each of the four directions around achip with the wafer being inclined in each step. However, since theangle of inclination (an angle of intersection) of a side wall surfaceof the trench, into which surface ion implantation is carried out,respect to the bottom surface of the wafer is large, about 125.3°, ionimplantation can be carried out without inclining the wafer (at a tiltangle of 0° (vertically)). In this case, only one time ion implantationis sufficient, so that the process can be simplified.

In forming a trench by dry etching in the related art, a high aspectratio of the trench caused reduction in an effective dose, a loss ofdose due to presence of a screen oxide film, a loss of dose due toreflection and re-emission of an ion beam, and reduction in an effectiveprojected range. In the example of formation of the trench according tothe present invention, however, no such problems are developed becausethe angle of inclination of the side wall surface of the trench to thebottom surface of the wafer is relatively large, about 125.3°, and theaspect ratio is small. Furthermore, the small aspect ratio facilitatesremoval of chemicals and residues in the trench, which is largelyeffective in enhancing yield and improving reliability. The angle ofinclination of the V-shaped trench is fixed at 54.7° (an angle ofintersection of the top surface of the wafer) as the (100) plane and the{111} plane at which etching is stopped. This causes no variation in theangle of inclination of the side wall. Hence, variations in a dose andranges of implanted ions at ion implantation become far small.

As an example, the case is given in which boron ions are implanted at1×10¹⁵ cm⁻²/100 keV at a tilt angle of 0°. A temperature and a time forlow temperature annealing carried out after the ion implantation arethose which exert no influence on the emitter electrode and thecollector electrode having been already formed (at an annealingtemperature of 400° C. for an annealing time of 5 hours, for example).Moreover, the flatness of 1 nm Ra of the side wall surface of the trenchcan ensure formation of the isolation layer with a diffusion depth of 1μm.

Subsequently, the thin semiconductor wafer, affixed onto a supportingsubstrate with the double-sided adhesive tape, is heated to remove thethermal foaming tape from the thin semiconductor wafer, by which thesemiconductor wafer is separated from the double-sided adhesive tapeaffixed to the supporting substrate. Moreover, by irradiating the UVtape with ultraviolet (UV) light, the UV tape is removed from thesupporting substrate, by which the double-sided adhesive tape isseparated from the supporting substrate for reusing the supportingsubstrate. Here, the removal of the thermal foaming tape is carried outwith the supporting substrate put on a hot plate at a raised temperatureof the order of 130° C. while being made to come under the semiconductorwafer (to come on a hot plate surface). Since the thickness left at thebottom of the trench is only several micrometers as a total of thethicknesses of the passivation layer 27 and the collector electrode 29,the thermal foaming tape removal from the semiconductor wafersimultaneously allows the semiconductor wafer to be separated intochips. When the wafer fails to break into separated chips, the leftcoupling section can be preferably cut by such measures as a high-energylaser beam irradiation. Thus, an IGBT chip is formed which is assembledin a package (not illustrated) to be completed as a reverse-blockingIGBT.

FIG. 6A is a characteristic diagram showing a relationship between anangle of inclination of the side wall of a trench and a relative ionimplantation time in which the ion implantation time when the angle ofinclination of the side wall is 0° is taken as unity. It is known fromFIG. 6A that, in a trench formed by ordinary dry etching with a sidewall having a large angle of inclination of 80° or more to the topsurface of the wafer, a very long implantation time is required when anion beam is incident perpendicularly on a wafer.

In Example 1 according to the present invention, a laser beam isoperated for activating an ion-implanted layer rather than operated in awork mode. The laser annealing is to be carried out by irradiating theion-implanted layer with a laser beam having such an irradiation energydensity as to make a concentration of implanted dopant ions be at asaturated state. In particular, with a thin wafer device with asubstrate thickness of 200 μm or less, by irradiating the ion-implantedlayer with a laser beam having such an irradiation energy density as tomake the concentration of implanted dopant ions be at the saturatedstate, the entire surface of the side wall of the trench can besufficiently activated.

Since a laser is a kind of light, an irradiation energy density on aplane, inclined at an angle of inclination θ to a plane perpendicular toan incident laser beam, is reduced to a value in which, like theirradiation energy density due to a normal light, the irradiation energydensity on the plane perpendicular to the incident laser beam ismultiplied by cos θ. Therefore, if a plane perpendicular to an incidentlaser beam (θ=0°) is activated by the laser beam, when a side wall of atrench inclined at an angle of inclination of θ is activated by a laserbeam irradiation, the energy of the laser beam is increased by a factorof 1/cos θ. This is to enable a plane inclined at any angle ofinclination θ to be activated.

However, when the area to be annealed includes a planar areaperpendicular to the laser beam in addition to the inclined side wall ofthe trench, the planar area or an area irradiated with an excessiveirradiation energy density is brought into a state in which a crystal ismolten. Otherwise, a work mark appears on a silicon substrate or thesilicon substrate is brought into a state of being processed.

Referring to FIG. 1, in Example 1, the manufacturing process of each ofthe trenches is the same as that previously explained. In the previousexplanation, the angle of inclination of the V-shaped trench was fixedat 54.7° of an angle of intersection of the (100) plane and the {111}plane at which etching is stopped. This caused no variation in the angleof inclination of the side wall. Hence, variations in a dose and rangesof implanted ions at ion implantation became far small. However, thedirections of crystal planes and the processed planes of wafers beingused are not always common to the wafers. In particular, in devices andactuators to which an MEMS (Micro-Electro-Mechanical System) technologyis applied, the number of processing making use of planes with variousdirections of crystal planes tends to increase.

In Example 1, boron ions were implanted at 1×10¹⁵ cm⁻²/50 keV at a tiltangle of 7°. Next, laser annealing was carried out with a YAG2ωdouble-pulse laser (with a total irradiation energy density of 3 J/cm²from two laser units (1.5 J/cm²+1.5 J/cm²), an wavelength of 532 nm anda delay time of 300 ns between the two laser units). At ionimplantation, an opening of a mask is ideally provided only at a trenchsection. While, at laser irradiation, the irradiation is ideally carriedout only at a section where the trench is formed.

In the arrangement shown in FIG. 1, on the surface of a semiconductorwafer 30, several trenches were formed in which angles of inclination θof side walls to the wafer surface were varied from one another. Thusformed trenches were irradiated with a laser beam and their respectiveannealing conditions were studied. In trenches 31, 32, 33, 34, 35, 36and 37, the angles of inclination θ are given as θ=55°, θ=60°, θ=65°,θ=70°, θ=75°, θ=80° and θ=85°, respectively. Reference numerals 38 and39 designate the direction of irradiation with the laser beam and thedirection of scanning with the laser beam, respectively.

FIG. 2A is a characteristic diagram showing impurity concentrationprofiles measured by an SR (Spreading Resistance) method, depending onthe angle of inclination of the side wall of the trench. Since an actualreverse-blocking IGBT device has a wafer thickness of the order of only200 μm, in the measurement, the semiconductor wafer 30 having a heightof 10 mm or less was prepared to form the trenches 31 to 37. The focalpoint of the laser beam was made positioned at the bottom of each trenchand the SR concentration profile was measured at a position 1 mm or lessabove the bottom of each trench. Profiles with numerals 31 to 37 arethose in the side walls of the trenches 31 to 37, respectively, and theprofile with numeral 40 is that in the flat plane as the surface of thesemiconductor wafer 30 perpendicular to the laser beam.

From the diagram, it is known that in the surface 40 with θ=0°, thetrench 31 with θ=55° and the trench 32 with θ=60°, high impurityconcentrations were obtained with the peak of each concentration profileexceeding 1×10¹⁹ cm⁻³, and that even in the trench 33 with θ=65°, a highimpurity concentration was obtained with the peak of concentrationprofile exceeding 5×10¹⁸ cm⁻³. Moreover, it is also known that intrenches from the trench 31 with θ=55° to the trench 34 with θ=70°,peaks of their impurity concentrations exceed 1×10¹⁷ cm⁻³, which aresufficient as concentrations for controlling expansion of a depletionlayer in a reverse-blocking IGBT. In the experiments carried out thistime, ions were implanted down to the depth of the order of 0.35 μm.However, by increasing implantation energy at ion implantation asnecessary, an ion implantation condition hard to be affected bycontamination and flaws can be determined.

FIG. 2B is a characteristic diagram showing SR impurity concentrationprofiles measured at several positions on the inclined side wall of atrench away from the focal point of a laser beam set at the bottom ofthe trench. With an angle of inclination θ taken as 55°, the focal pointwas set at the bottom “a” of the trench, a position on the side wall ofthe trench 1 mm away from the bottom was taken as a point “b” and aposition 2.5 mm away from the bottom was taken as a point “c”.Implantation energies of ions were taken as 50 keV, 100 keV and 150 keVwith a boron dose taken as 1×10¹⁵ cm⁻² and a laser irradiation conditionmade the same as above.

In FIG. 2B, the concentration profiles with numerals 41, 42 and 43 arethose obtained at the point “b” (the position 1 mm away from the focalpoint) with the implantation energies of 50 keV, 100 keV and 150 keV,respectively. The concentration profiles with numerals 44, 45 and 46 arethose obtained at the point “c” (the position 2.5 mm away from the focalpoint) with the implantation energies of 50 keV, 100 keV and 150 keV,respectively. It is known from FIG. 2B that with implantation energy upto 150 keV, even at a position 1 mm away from the focal point of thelaser beam, high concentrations with the peak concentration of 1×10¹⁹cm⁻³ or more can be obtained. Moreover, it is known that even withimplantation energy of 50 keV, a peak concentration of 1×10¹⁷ cm⁻³ ormore is obtained even at a position 2.5 mm away from the focal point.Therefore, it is known that in a wafer with a thickness equal to or lessthan the above distance (for example, a thickness on the order of 200 μmas being applied to the reverse-blocking IGBT according to theinvention), the impurity concentration in the inclined side wall of thetrench has no problem in being made as an impurity concentration forforming an isolation layer.

FIG. 6B is a characteristic diagram showing a relationship between theangle of inclination θ of the side wall of the trench and a rate ofcracking of chips. When annealing is carried out by irradiating the sidewall of the trench with a laser beam, the smaller the angle ofinclination θ is (or larger the supplemental angle 180°-θ is), the morean irradiated surface is activated. That is, the efficiency ofactivation is the best for the angle of inclination θ given as θ=0°(when the laser beam is projected from the direction perpendicular tothe flat plane of the wafer). However, when the case of separating thewafer into individual chips is taken into consideration, at θ<30°, theedge of the chip is formed to have a considerably sharp acute angle,which can cause cracking of the chip. Even though a region with a highimpurity concentration is formed at the edge having a sharp acute angleto be provided as a field-stopping layer, the sharper the edge, the morethe position causing a crack comes closer to the central part of thechip. Thus, it becomes impossible to limit the cracking within thefield-stopping layer only. For the angle of inclination θ given asθ>30°, the rate of cracking of chips becomes 20% or less. Accordingly,the angle of inclination of the side wall of the trench relative to thetop surface of the wafer should be taken as 70° or less, and preferablytaken as between 30° and 70°.

Example 1 was explained as an example in which laser beam irradiation isapplied to activation of dopant ions implanted for forming an isolationlayer of a reverse-blocking IGBT device. In Example 2, since a laser isoriginally a kind of light, an irradiation energy density on a plane,inclined at an angle of inclination θ to a plane perpendicular to anincident laser beam, can be considered to be reduced basically on thecosine law to a value in which, like the irradiation energy density dueto a normal light, the irradiation energy density on the planeperpendicular to the incident laser beam is multiplied by cos θ.Therefore, on condition that a plane perpendicular to an incident laserbeam (θ=0°) is activated by the laser beam, when a side wall of a trenchinclined at an angle of inclination of θ is activated by a laser beamirradiation, the energy of the laser beam is made increased by a factorof 1/cos θ. This is to enable a plane inclined at any angle ofinclination θ to be activated.

For example, when a flat plane perpendicular to an incident laser beamcan be activated with an energy density of 1 J/cm², with an energydensity given by three times or more, i.e., 3 J/cm² or more, a planewith an angle of inclination θ up to 70.5° (θ=cos⁻¹(1/3)) is to becapable of being activated though losses of the laser beam due toreflection and transmission are not taken into consideration in theexpression. With an angle of inclination θ taken as 70° and an energydensity of a laser beam taken as 4 J/cm², the energy density on theirradiated inclined plane is 1.37 J/cm². When the focal point of thelaser beam is set at the bottom of the trench and energy density 4 J/cm²at the bottom is relatively taken as unity, the energy density 1.37J/cm² on the inclined plane is relatively 0.34, i.e., 34%. Thus, whenthe energy density 4 J/cm² at the focal point (bottom) is relativelytaken as unity, the height of the trench is taken as that at which anenergy density of 35% or more of the energy density at the bottom can beensured.

In Example 2, calculation exhibits that with the irradiation energydensity taken as, for example, 4 J/cm² and 6 J/cm², the inclined planecan be activated at an angle of inclination θ up to θ=75.5° and θ=80.4°,respectively. In Example 3, however, as shown in FIG. 2C, which is acharacteristic diagram showing impurity concentration profiles in thedepth direction measured by irradiation energy density, high irradiationenergy densities affect impurity concentration profiles. FIG. 2C showsresults of measurements of impurity concentration profiles in which asurface, implanted with a high dose of boron ions with relatively lowimplantation energy such as 50 keV as in Example 1, was irradiated withlaser beams with respective irradiation energies taken as 4 J/cm²(profile 47), 3 J/cm² (profile 48), and 6 J/cm² (profile 49).Irradiation with the energy density of 4 J/cm² causes the boron ionimplanted surface to initiate melting, by which the profile 47 isexhibited as becoming box-shaped profile that is different from theprofile 48 obtained by irradiation with an energy density of 3 J/cm².Moreover, irradiation with the energy density of 6 J/cm² causes theirradiated surface to be completely molten to make a p boron implantedlayer with a thickness of the order of 0.3 μm disappear. Hence, theimpurity concentration on the surface side can be reduced. Therefore,the irradiation energy density must be determined to be on a level thatcauses no melting of an irradiated surface while being on cosine law.When a YAG2ω double-pulse laser is used, its irradiation energy densitymust be determined to be 4 J/cm² or below.

Moreover, in a reverse-blocking IGBT with the isolation layer formed bythe above isolation layer forming method, an adverse effect due tooxygen can be eliminated more than in an IGBT in which the isolationlayer is formed by a coating and diffusion method, so that a farexcellent rate of acceptable products (>90%) can be ensured.

FIG. 2D is a characteristic diagram showing results of carrierconcentration profile measurements respectively carried out about theflat plane section and the trench side wall section of a wafer with aV-shaped trench formed by wet anisotropic etching with an alkalinesolution. On the entire surface of a wafer with a V-shaped trench formedby wet anisotropic etching with an alkaline solution, boron ions wereimplanted with implantation energy of 45 keV and a dose of 1×10¹⁵ions/cm² at a tilt angle of 0° (with the ion beam made incidentperpendicularly to the wafer). Thereafter, the entire surface of thewafer was annealed by irradiating with a YLF2ω double-pulse laserannealing device with an irradiation energy of 1.4 J/cm²×2 and a delaytime of 300 nsec for the second pulsed laser irradiation. Since boronions as dopant ions implanted into the side wall of the trench areactivated by the laser annealing, an isolation layer can be obtainedwith an impurity concentration exceeding 1×10¹⁹ cm⁻³, and a depth on theorder of 1 μm. Thus obtained isolation layer, being harder to bedepleted than an isolation layer obtained in the case when lowtemperature annealing is used, can enhance a yield of the product thatmeets the reverse breakdown voltage specification. The reason that theimpurity concentration becomes higher than that in the case when lowtemperature annealing is used is that silicon can be madeinstantaneously molten and recrystallized at a very high speed ofseveral meters per second.

Furthermore, the laser annealing exerts influence of heat only on aregion near the irradiated surface. Therefore, no thermal damage isadded again to the device, so that the laser annealing is provided as agood method. Like in the boron ion implantation for forming thecollector diffused layer, the irradiation of the entire wafer surfaceenables the flat plane section and the trench side wall section to besimultaneously subjected to activation processing. Laser annealing canbe carried out by laser irradiation onto a part desired to be irradiatedby covering the other parts with masks, such as formed of SUS, or bypartially irradiation onto the desired parts. The partial irradiationlaser annealing is a method of partially carrying out annealing bymaking a laser beam carry out partial scanning or by controlling laserirradiation with a shutter, which can be opened and closed while laserbeam scanning is carried out. In a reverse-blocking IGBT, formation ofan isolation layer can be carried out by using the laser annealing whenelectrical characteristics such as breakdown voltages can be ensured ina wafer with a thickness down to 1 mm.

Here, in the laser annealing process, the laser annealing is carried outwith the irradiation being made to leave no work mark (in a state of nowork mode). An irradiation energy density at this time is 2 J/cm² orless per one unit, and with a laser beam of a YLF2ω laser or a YAG2ωlaser, an irradiation energy density is 0.25 to 5.0 J/cm². It isadequate to carry out a whole surface irradiation with such an energydensity with a delay time of irradiation between two units given as 0 to5000 ns. In the laser dicing process, the dicing is carried out with theirradiation being made to enter a work mode. For bringing theirradiation into the work mode, it is necessary only that theirradiation energy density be brought to be 2 J/cm² or more per oneunit. When cutting of a metal electrode with a thickness of severalmicrometers is carried out, an adequate irradiation energy density is onthe order of 3 J/cm² per one unit.

The examples according to the invention were described as being carriedout by using a YAG2ω double-pulse laser. However, a YLF2ω laser (at awavelength of 527 nm), a YLF2ω double-pulse laser, an excimer (such asXeF and XeCl) laser, a YAG3ω laser or a semiconductor laser can be usedwith their respective irradiation energy densities adjusted. Of theabove lasers, the XeCl laser (at a wavelength of 308 nm) with anirradiation energy density being 1.4 J/cm² or more forms a work mark.Therefore, the irradiation energy density of the XeCl laser must betaken as 1.4 J/cm² or less.

The ion implantation condition and the laser annealing condition areconditions for forming the collector diffused layer on the bottomsurface of the wafer. However, since an angle of inclination of thetrench to the top surface of the wafer is 54.7°/125.3°, a sufficientamount of dose of dopant ions is also introduced into the side wall ofthe trench. Moreover, energy absorption into the side wall at the laserannealing is also sufficient for activating dopant ions introduced intothe side wall. Namely, with conditions for ion implantation and laserannealing being the same as those for forming the p collector diffusedlayer on the flat bottom surface, formation of the p isolation diffusedlayer on the inclined surface of the side wall of the trench can becarried out simultaneously. Here, referring to FIG. 9, although the doseand range of dopant ions implanted into the side wall of the trench arereduced from unity to an amount of cos 54.7° (about 0.58) compared withthe dose and range of dopant ions implanted into the flat bottomsurface, they are sufficient for achieving a reverse-blocking capabilityin a reverse-blocking IGBT and causes no problem. The angle ofinclination of 54.7°/125.3° of the side wall of the trench enablesactivation of the isolation layer and the collector layer to be carriedout together also by laser annealing. This is effective in reducing costand enhancing the rate of acceptable products.

By carrying out the above-described laser annealing treatment, boronatoms are activated and introduced into the isolation layer on the sidewall of the trench on the bottom surface and the collector layer in theflat plane section of the bottom surface. The activation of theisolation layer and the collector layer can be also carried outsimultaneously with activation treatment of the dopant atoms carried outby furnace annealing at a relatively low temperature (up to 550° C.).

Since the isolation diffused layer and the collector diffused layer areformed from the bottom surface side after the MOS gate structure on thetop surface side is formed, the process step of forming the collectormetal electrode on the bottom surface can be carried out with the trenchmade left open without being filled. This allows the side wall of thetrench to also have the metal film deposited which can be made tofunction as a protection film of the side wall of the trench.

The formation of the trench, having a relatively large angle ofinclination, about 125.3°, relative to the bottom surface side, by wetanisotropic etching with an alkaline solution is effective in enablingsignificant reduction of spacing between chips, i.e., a device pitch.Furthermore, the trench reaching the top surface makes the dicingprocess unnecessary to allow reduction of defects due to dicing.

FIGS. 4A to 4C are schematic cross-sectional views showing comparison ofthe present invention and the related art for reducing a device pitch.FIG. 4A is a schematic cross-sectional view showing a width of anisolation layer provided by coating and diffusion in a wafer in therelated art, where the width 31 of the isolation layer is 500 μm. FIG.4B is a schematic cross-sectional view showing the case of forming aV-shaped trench from the top surface of a wafer, where the width 31 ofthe isolation layer is 300 μm. FIG. 4C is a schematic cross-sectionalview showing the case of forming a V-shaped trench from the bottomsurface of a wafer, in which the width 31 of the isolation layer is 0 to30 μm. For defining the top surface and the bottom surface of the wafer,a collector diffused layer 7 and an active region 24 are shownschematically. Reference numeral 32 denotes a dicing line when the waferis separated into chips.

In the wafer shown in FIG. 4C, with variation in the wafer thickness,that is, variation caused in processing such as back grinding, beingzero, the spacing between chips adjacent to each other can be made zero.In actuality, however, as shown in FIG. 4D, which is a partialcross-sectional view illustrating effect of variation in the thicknessof a wafer on spacing between chips, variation of ±10 μm can occur inback grinding to the thickness of a wafer set at 200 μm. Namely, thethickness of the wafer varies in the range from 190 μm to 210 μm, whichcauses variation of 15 μm in the spacing between chips. Therefore, adevice pitch taken with a margin of approximately 30 μm effectivelyreduces a rate of defective products and still provides the largestpossible number of chips. The state is schematically shown in FIGS.7A-7D. FIGS. 7A and 7B illustrate a plan view and a cross-sectional viewin which the spacing between chips is increased in anticipation ofvariations caused in alignment and in back grinding. FIGS. 7C and 7Dillustrate a plan view and a cross-sectional view in the spacing betweenchips is lessened with such variations being reduced.

FIGS. 8A-8C are schematic cross-sectional views for explainingresistance to occurrence of cracking and the degree of tendency ofcausing electric field concentration at the edge of the bottom surfacewith chips in related art in comparison with the chip formed accordingto the present invention. Cross sections of the ends of the separatedchips are schematically shown in FIGS. 8A-8C, corresponding to thewafers shown in FIGS. 4A-4C, respectively. In a reverse-blocking IGBTchip according to related art shown in FIG. 8A separated at an isolationlayer formed by coating and diffusion, the edge of the bottom surfaceforms a right angle. In a reverse-blocking IGBT chip according torelated art shown in FIG. 8B separated at a V-shaped trench formed fromthe top surface, the edge of the bottom surface forms an acute angle.This is liable to cause defective chips due to cracking and tends tocause high electric field concentration at the edges of the bottomsurfaces.

In comparison, in a reverse-blocking IGBT chip separated at a V-shapedtrench formed from the bottom surface according to the present inventionshown in FIG. 8C, the edge of the bottom surface forms an obtuse angleof about 125.3°. This significantly reduces production of defectivechips due to cracking at the edges of the bottom surfaces. Moreover, thedegree of tendency of causing electric field concentration at the edgeof the bottom surface is reduced to produce an effect of enhancing abreakdown voltage. In return for the edge of the bottom surface formingan obtuse angle of 125.3°, the edge of the top surface forms an acuteangle of 54.7° to raise the possibility of causing a defect due todamage. The edge of the top surface, however, is formed as an endsection of the surface of a voltage withstanding structure with ap-layer of a high impurity concentration formed at an early stage ofIGBT fabrication process by introduction of dopant such as ionimplantation and diffusion on the IGBT surface. Therefore, devicecharacteristics are not largely affected even though defects due to flawor cracking are possible. The p-layer 33 with the high impurityconcentration is sufficiently provided with a depth of approximately5-30 μm, which can be formed by diffusion carried out beforehand in theprevious processing step.

For the etching mask used in the wet anisotropic etching with analkaline solution, a silicon oxide (SiO₂) film or a silicon nitride(Si₃N₄) film can be used. In wet anisotropic etching of silicon with analkaline solution, very high etching mask selectivity is obtained.Therefore, even with an SiO₂ film formed by CVD being generally inferiorto a thermal oxide film in film quality (resistance of mask), asufficient etching mask selectivity can be obtained. Thus, in additionto a silicon oxide film or a silicon nitride film formed by plasma CVDor normal pressure CVD, material such as an SOG (spin-on-glass) film,alkali resistant resin or alkali resistant resist can be used as etchingmask material. This is effective in that occurrence of crystallinedefects and thermal donors is significantly inhibited which was causedby heat treatment at an elevated temperature for a long time due to thecoating and diffusion which became the problems in the related art.Moreover, a temperature for forming the etching mask material can belowered, at a room temperature to 500° C. This enables the trenchformation processing to be carried out at a later stage of the waferformation process without causing thermal damage to an aluminumelectrode formed on the top surface.

In the formation of a trench by dry etching as prior art, a very smalletching rate, namely 2 μm/min, and processing usually carried out by asingle wafer processing etching system make the formation of the trenchtake a very long time. While, in the wet anisotropic etching with analkaline solution, a method referred to as batch processing is possiblethat processes several to tens of wafers at a time and etching rate canalso be set very high, which is very largely effective in reducing thelead time and the cost. Since the wet etching is usually carried out at100° C. or below, even though a metal with relatively low melting point,such as aluminum or a material that is not heat resistant, is alreadyformed on a wafer, the etching exerts no influence on them.

The angle of inclination of the side wall of the trench is relativelylarge, about 125.3° relative to the bottom surface side in comparisonwith the angle of inclination of the trench formed by dry etching. Thiscan inhibit reduction in an effective dose, a loss of a dose due topresence of a screen oxide film, a loss of a dose due to reflection andre-emission of an ion beam, and reduction in an effective projectedrange that occur in the case when impurity ions are implantedperpendicularly to the wafer. Furthermore, the large angle ofinclination of the trench allows sufficient energy to be absorbed in theside wall of the trench to make it possible to activate dopant even inlaser annealing in which a laser beam is made incident perpendicularlyto the wafer. Since ion implantation and heat treatment to the collectorlayer and the isolation layer can be carried out at the same time, aneffect is produced in which a cost is reduced, the number of processsteps is reduced and a rate of acceptable products is increased.

After the isolation layer and the collector layer are formed, theprocess step of forming the bottom surface collector metal electrode iscarried out. This allows the side wall of the trench to also have themetal film deposited thereon. The metal film functions as a protectionfilm of the side wall of the trench to thereby reduce occurrence ofdefects due to cracking and inclusion of contaminant, which is effectivein largely enhancing a rate of acceptable products.

FIG. 3 schematically illustrates the manufacturing steps of thereverse-blocking IGBT according to the invention. First, a MOS gatestructure 18 is formed on the top surface (first principal) side of awafer 17 (step (a)). Then, the face of the bottom surface (secondprincipal) side of the wafer 17 is ground thin and is further subjectedto etching (step (b)). On the first principal surface side of thethinned wafer 17, a supporting substrate 19, such as a silica glasswafer, is affixed (step (c)). The affixed supporting substrate 19 isremarkably effective for the thinned wafer 17. This is because thesupporting substrate 19 affixed to the wafer 17 prevents the wafer 17itself from warping, deflecting, and cracking. Further, it additionallyprevents chips integrally formed to the wafer 17 from being separatedinto individual ones after etching, and further prevents the MOS gatestructure formed on the first principal surface (the top surface) of thewafer 17 from being eroded by etching solution. Thereafter, the affixedwafer 17 and supporting substrate 19 are turned over (step (d)), and thebottom surface etching mask 20 is formed on the second principal surfaceof the wafer 17 by using a double-sided aligner (step (e)). Thealignment with a double-sided aligner is generally carried out so that amask pattern is aligned on the second principal surface with referenceto the pattern on the first principal surface. Therefore, the supportingsubstrate 19 and an adhesive for bonding the supporting substrate 19 andthe wafer 17 are desirably transparent.

The bottom surface etching mask 20 is formed by the double-sided alignerafter the supporting substrate 19 is affixed to the wafer 17. However,when alignment can be carried out about the unsupported thin wafer 17without causing any large warping and deflection, the supportingsubstrate 19 can be affixed to the wafer 17 after the mask forming stepis finished. After the bottom surface etching mask 20 has been formed, atrench 21 is formed by alkaline etching (step (f)). Next, the bottomsurface etching mask 20 is removed (step (g)), boron ion implantation iscarried out (step (h)), and laser annealing is carried out to form a pisolation layer and a p diffused layer 22 (step (i)). Then, metal filmsfor the collector electrode 23 are formed (step (j)), and the wafer 17is finally separated to be provided as individual chips (step (k)).

The method according to the invention can reduce contamination of thewafer in comparison with the related method by which, after the trenchis formed, the bottom surface of a thick semiconductor wafer is groundand then a bottom surface structure is formed. Moreover, no degradationin characteristics due to oxygen occurs unlike the isolation layerformed by the related coating and diffusion method. Hence, a high rateof acceptable products of 90% or more can be stably obtained. Inaddition, unlike the related manufacturing process, the step of fillingthe trench with a reinforcing material can be omitted, which step wascarried out for the semiconductor wafer to be diced into chips. Hence, areverse-blocking IGBT can be provided at a low cost with highreliability.

The flatness of the side wall of the trench is excellent as 1 nm Ra.Therefore, even with the isolation layer having a diffusion depth of theorder of 1 μm with an impurity concentration of the order of 10¹⁸ cm⁻³,a depletion layer can be surely stopped. Although boron was used as adopant for forming the above-described isolation layer, aluminum can bealso used. In addition, although the UV tape was used for the adhesivetape on the side of the supporting substrate, a removable tape used inan ordinary grinding process (a back-grinding process) can be used,which can be removed from the supporting substrate by peeling (pullingoff).

The angle of inclination of the side wall of the trench to the secondprincipal surface is large in comparison with that of the trench formedby dry etching. This can reduce adverse effects of a related method inion implantation, namely those such as reduction in an effective dose, aloss of a dose due to presence of a screen oxide film, a loss of a dosedue to reflection and re-emission of an ion beam, and reduction in aneffective projected range. Furthermore, with the angle of inclination ofthe side wall of the trench to the second principal surface being large,a chemical residue in the trench can be easily removed, which is largelyeffective in enhancing yield and in improving reliability.

Furthermore, by carrying out an annealing process after the ionimplantation with laser annealing, the impurity in the isolation layercan be activated with its temperature instantaneously elevated up tothat near the melting point of silicon. Hence, dopant (p-type dopantsuch as B or Al, for example) ions implanted for forming the isolationlayer can be activated more highly than dopant ions annealed by lowtemperature furnace annealing carried out at a temperature of 500° C. orless. At this time, only the region down to several micrometers from thesurface is activated, so that no influence is exerted on and no thermaldamage occurs in the already formed electrode.

With the manufacturing method of the semiconductor device according tothe invention, an isolation layer can be formed without carrying outhigh temperature and long time diffusion processing and long timeoxidation processing. Moreover, a heat treatment time for forming theisolation layer can be reduced. Furthermore, defects due to crystaldefects and defects due to flaws or chippings can be reduced. Inaddition, process steps for dicing, CMP (Chemical Mechanical Polishing),backfilling with an epitaxial layer, and coating and diffusion can bemade unnecessary. Therefore, a production cost can be significantlyreduced compared with that in the related art. Further, no OSF and nooxygen donor are produced to make it expectable to reduce a reverse leakcurrent and to enhance a breakdown voltage. Therefore, the invention iseffective in that a highly reliable semiconductor device can be obtainedat a low cost. Moreover, besides being effective for a reverse-blockingIGBT, the technology in the manufacturing method according to theinvention is also effective for actuators and sensors that necessitatehaving trench structures similar to that of the IGBT formed inactivating introduced dopant in the trench surface.

In the wet anisotropic etching with an alkaline solution, the etchingmask can be thinned by forming the mask with a silicon oxide film or asilicon nitride (Si₃N₄) film having very high mask selectivity. Forexample, when a silicon oxide film is used for an etching mask and apotassium hydroxide (KOH) aqueous solution is used for an etchingsolution, mask selectivity is very large as much as 1:350 to 1:500. Thisallows a silicon oxide film as a mask oxide film to be very thin.Therefore, when a mask oxide film is formed by thermal oxidation, anoxidation temperature can be lowered and an oxidation time can besignificantly shortened. This can reduce a problem of increasing a leadtime and a problem of causing crystalline defects due to oxygenintroduction at oxidation that are experienced in related devices andmethods.

Moreover, a silicon oxide film formed by chemical vapor deposition (CVD)has sufficient mask selectivity as an etching mask though such a siliconoxide film is a little inferior to a thermal oxide film in film quality(resistance of mask). Thus, a TEOS (Tetraethylorthosilicate) film or asilicon nitride film formed by reduced pressure CVD or plasma-assistedCVD can be used as an etching mask. In this case, a temperature forforming the film with CVD is as low as 200 to 680° C., so that in thelatter part of a wafer forming process, namely after formation of a MOSgate structure, after formation of an emitter electrode or afterformation of a surface protection film, a trench for forming anisolation layer can be formed.

In wet anisotropic etching with an alkaline solution, an etching ratecan be determined very high. For example, in the case of carrying outetching at 110° C. by using a potassium hydroxide aqueous solution witha concentration of 54 wt %, the etching rate is approximately 8 μm/min.In addition, in wet etching, the etching can be carried out by a systemreferred to as a batch processing system in which several to tens ofwafers can be simultaneously processed, which is very largely effectivein lead time reduction and cost reduction.

Moreover, in the wet anisotropic etching with an alkaline solution, anetching temperature is taken at 200° C. or less. This makes a thermalbudget so small as to exert no influence on a dopant profile in theactive region. Furthermore, even though structures of metals withcomparatively low melting points such as aluminum (Al) or ofnon-heat-resistant materials are formed on the wafer before the trenchis formed by the wet anisotropic etching, no influence is exerted on thestructures by carrying out the etching.

Moreover, by forming the trench by wet anisotropic etching with analkaline solution and by thereafter carrying out implantation of boronions into the side wall of the trench, a heat treatment temperature canbe made lower than that in a related method and a heat treatment timecan be made shorter than that in a related method. This allows an effectof reduction in the lead time at formation of the isolation layer and anaccompanied improvement in the rate of acceptable products. In addition,the taper angle of the side wall of the trench is very large comparedwith that of the trench formed by dry etching. This can reduce adverseeffects of a related method in ion implantation, namely those such asreduction in an effective dose, a loss of a dose due to presence of ascreen oxide film, a loss of a dose due to reflection and re-emission ofan ion beam, and reduction in an effective projected range. Furthermore,with the taper angle of the side wall of the trench being very large, achemical and a residue in the trench can be easily removed, which islargely effective in enhancing yield and in improving reliability.

In addition, in the wet anisotropic etching with an alkaline solution,when a mask alignment is arranged in the [110] direction as a crystalorientation equivalent to the <110> direction by using a silicon waferwith the (100) plane, for example, taken as the principal surface, thetaper angle of the side wall of the trench is fixed at 125.3°. Thisgives the effect of making process dependent variation small.Furthermore, by the side wall of the trench fixed at the taper angle of125.3°, at the time when the etching in progress comes to provide thetrench with a V-shaped cross section, the progress of the etchingnaturally stopping. In other words, the depth of the trench isdetermined by the opening width of the etching mask. This is effectivein making variation in the depth of the trench very smaller than thatexperienced in a related method.

Furthermore, a thin semiconductor wafer, formed with a top surfacestructure and a bottom surface structure for forming a reverse-blockingsemiconductor chip, is affixed to a supporting substrate, a trench to bea scribing line is formed on the thin semiconductor wafer, an isolationlayer is formed on the side face of the trench, and the semiconductorwafer is removed from the supporting substrate to be provided assemiconductor chips. This allows a dicing process as was carried out inthe related method to be omitted. Moreover, no coating and diffusionmethod is used in forming the isolation layer, so that an adverse effectsuch as degradation in characteristics due to oxygen can be reduced.Therefore, a reverse-blocking semiconductor device with high reliabilitycan be provided at a reduced cost. Furthermore, by using low temperatureannealing or laser annealing for activating the dopant in the isolationlayer, the isolation layer can be provided as being uniform and shallowin diffusion depth. This allows the area occupied by the isolation layerin the semiconductor chip to be made smaller than the area provided bythe related coating and diffusion method, which enables reduction in adevice pitch.

For affixing the device to the supporting substrate, a double-sidedadhesive tape with a thermal foaming tape and a UV tape bonded togetheris used. The thermal forming tape is affixed onto the device surface andthe UV tape is affixed to the supporting substrate. This makes thedouble-sided adhesive tape foamed to be easily removed away from thedevice by heating treatment at a relatively low temperature between 80°C. and 200° C. (for example, at 130° C.).

With the semiconductor device and the manufacturing method of thesemiconductor device according to the invention, the trench is formedfrom the side of the bottom surface as the second principal surface ofthe silicon semiconductor substrate by wet anisotropic etching with analkaline solution and impurity ions are implanted into the side wall ofthe trench. This allows the isolation layer to be formed withoutcarrying out high temperature and long time diffusion processing andlong time oxidation processing. Moreover, the isolation layer is formedwith the semiconductor substrate affixed to a supporting substrate tothereby allow the isolation layer, connected to the diffused layer onthe side of a second principal surface, to be easily formed withoutcausing the wafer to be separated into individual chips flaking off. Thewidth of the isolation layer can be provided as 30 μm compared with 300μm in a related device and a heat treatment time for forming theisolation layer can be also reduced to 300 nanoseconds from 230 hoursfor a related device. Furthermore, defects due to crystalline defectsand defects due to flaws or chippings can be reduced. In addition,process steps for dicing, CMP (Chemical Mechanical Polishing),backfilling with an epitaxial layer, and coating and diffusion can bemade unnecessary. Therefore, a production cost can be significantlyreduced compared with that in the related art. Further, no OSF and nooxygen donor are produced and a p-n junction between the isolation layerand the silicon semiconductor substrate forms an obtuse angle to thebottom surface as the second principle surface of the siliconsemiconductor substrate to provide a structure in which a bottom surfaceedge of a chip reduces electric field concentration. Thus, reduction ina reverse leak current and enhancement in a breakdown voltage can bealso expected. Therefore, the invention is effective in that a highlyreliable semiconductor device with a small device pitch and a chip sizecan be obtained at a low cost.

The present invention is not limited to the above-explained embodimentsbut can be variously modified. In the examples, a trench was formed fromthe bottom surface of a substrate, but the trench can be formed from thetop surface of the substrate. Moreover, the first conductivity type wastaken as an n-type and the second conductivity type was taken as ap-type. The invention, however, is also valid even though theconductivity types are reversed. Furthermore, the invention can bevalidly applicable not only to the reverse-blocking IGBT but also toother kinds of reverse-blocking devices and bi-directional devices, orto semiconductor devices such as MOSFETs, bipolar transistors and MOSthyristors, each of whose manufacturing processes is accompanied byformation of an isolation layer, or actuators, sensors or solar cellsusing an MEMS technology.

As explained in the foregoing, the semiconductor devices and the methodsof manufacturing the devices according to the invention are useful forpower semiconductor devices used for a system such as a power conversionsystem, and in particular, are suited for bi-directional devices orreverse-blocking devices, or actuators or sensors using an MEMStechnology.

While the present invention has been particularly shown and describedwith reference to particular embodiments, it will be understood by thoseskilled in the art that the foregoing and other changes in form anddetails can be made therein without departing from the spirit and scopeof the present invention. All modifications and equivalents attainableby one versed in the art from the present disclosure within the scopeand spirit of the present invention are to be included as furtherembodiments of the present invention. The scope of the present inventionaccordingly is to be defined as set forth in the appended claims.

This application is based on, and claims priority to, JP PA 2005-088479,filed on 25 Mar. 2005, and JP PA 2005-090662, filed on 28 Mar. 2005. Thedisclosures of the priority applications, in their entirety, includingthe drawings, claims, and the specifications thereof, are incorporatedherein by reference.

1. A method of manufacturing a semiconductor device, the methodcomprising the steps of: forming a trench having a substantiallyV-shaped or trapezoidal-shape cross section, in a second principalsurface of a first conductivity type semiconductor substrate having afirst principal surface and the second principal surface; introducing asecond conductivity type impurity into a side wall of the trench; andactivating the impurity by laser irradiation, wherein the side wall ofthe trench is formed so as to have an angle of inclination of 30°-70°relative to the first principal surface, wherein the laser irradiationis carried out with a laser beam made incident onto the side wall of thetrench, with the laser beam made being incident onto the secondprincipal surface of the semiconductor substrate at an angle of incidentof ±30° to a normal to the second principal surface, and wherein thelaser irradiation is carried out with a focal point of the laser beamincident onto the side wall of the trench so that no molten crystal markor work mark appears on the second principal surface of thesemiconductor substrate.
 2. The method of manufacturing a semiconductordevice according to claim 1, wherein a crystal face of each of the firstand second principal surfaces of the semiconductor substrate is a {100}plane, and a crystal face of the side wall of the trench is a {111}plane.
 3. A method of manufacturing a semiconductor device, the methodcomprising the steps of: forming a trench having a substantiallyV-shaped or trapezoidal-shape cross section, in a second principalsurface of a first conductivity type semiconductor substrate having afirst principal surface and the second principal surface; introducing asecond conductivity type impurity into a side wall of the trench; andactivating the impurity by laser irradiation, wherein the side wall ofthe trench is formed so as to have an angle of inclination of 30°-70°relative to the first principal surface, wherein a crystal face of eachof the first and second principal surfaces of the semiconductorsubstrate is a {100} plane, and a crystal face of the side wall of thetrench is a {111} plane.